1. Field
The present invention relates to a lithographic apparatus and a method to detect a slip of a patterning device in such lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a scanning type lithographic apparatus, a mask (or patterning device) is carried by a support, also referred to as a mask table or patterning device table. While generating a pattern on a target portion of a substrate, the mask table performs scanning movements along a line of movement, in a single scan direction or scanning in both (i.e. opposite) directions along the line of movement. When a reversal of direction takes place, the mask table is decelerated and accelerated between the successive scanning movements. Also, the mask table is accelerated and decelerated before and after each scanning movement in a specific direction. Conventionally, the scanning movements are made with constant velocity. However, the scanning movements may also at least partly be made with varying velocity, e.g. the movements including at least part of the deceleration and/or acceleration phases.
The mask table supports, i.e. bears the weight of, the mask. It holds the mask in a manner that depends on the orientation of the mask, the design of the lithographic apparatus, and other conditions, such as for example whether or not the mask is held in a vacuum environment. The mask table may include a frame or a table, for example, which may be fixed or movable as required. The mask table (and its control system) may ensure that the mask is at a desired position, for example with respect to the projection system.
The mask is coupled to the mask table through a clamp. Conventionally, the mask is coupled to the mask table through a vacuum clamp which may be implemented as one or more vacuum pads provided on the mask table, where at least a part of a circumferential area of the mask is held onto the vacuum pads. By the clamp, a normal force between adjacent surfaces of the mask and the mask table is generated, resulting in a friction between contacting surfaces of the mask and the mask table. The vacuum pads include one or more openings coupled to a gas discharge and supply system. Instead of a vacuum coupling between the mask and the mask table, other forms of couplings based on friction between the mask and the mask table are conceivable, such as electrostatic or mechanical clamping techniques to hold the mask against the mask table.
In an ongoing development, increasing throughput requirements placed on lithographic apparatus lead to increasing scanning velocities. Consequently, deceleration and acceleration of the mask table increase. In the deceleration and acceleration phases, increased inertia forces act on the mask table and on the mask.
It is known that inertia forces acting on the mask table and the mask may lead to slip of the mask and the mask table relative to each other. The slip usually is in the order of nanometers. For relatively low decelerations and accelerations, the slip has appeared to be low and approximately constant over time, changing its direction with each deceleration/acceleration phase. In such circumstances, the slip may be ignored if it is sufficiently low, or the slip may be compensated by suitably calibrating a positioning device controlling the position (and hence, the movement) of the mask table and/or the substrate stage.
However, with increasing decelerations and accelerations, the slip occurring between the mask and the mask table increases, and becomes variable and unpredictable. Factors influencing the amount of slip may include, but may not be limited to, a flatness and roughness of the surfaces of the mask and the mask table engaging each other, a humidity of the atmosphere(s) in which the mask and the mask table are handled, a contamination of the mask or the mask table, and a degree of vacuum when the mask is held on the mask table by vacuum pads. Thus, a calibration of the positioning device will not lead to a correct positioning of the mask table and/or the substrate stage under the circumstances of high inertia forces.
Not only the speed of movement and acceleration of the mask table may tend to increase, also, accuracy requirements on the lithographic apparatus may become more stringent. Therefore, slip of the mask becomes less tolerable, as slip of the mask may result in a position error of the pattern projected onto the substrate.